# \large\bf $B_c \to (J/\Psi,\,\eta_c)\tau\nu$ semileptonic decays within Standard model and beyond

## TL;DR

This paper investigates semileptonic B_c decays to J/ψ and η_c within and beyond the Standard Model, analyzing potential new physics effects on decay observables due to existing anomalies in B meson decays.

## Contribution

It provides theoretical predictions for B_c to J/ψ and η_c decays using a general effective Lagrangian, exploring new physics impacts on measurable quantities.

## Key findings

- Predicted ratios of branching fractions under various new physics scenarios.
- Estimated tau polarization fractions and forward-backward asymmetries.
- Highlighted potential deviations from Standard Model expectations.

## Abstract

Deviations from the standard model prediction have been observed not only in $b \to c$ charged current interactions but also in $b \to s$ flavor changing neutral current interactions. In particular, the deviation observed in the measured ratio of branching fractions $R_D = \mathcal B(B \to D\tau\nu)/\mathcal B(B \to D\,l\,\nu)$ and $R_{D^{\ast}} =\mathcal B(B \to D^{\ast}\tau\nu)/\mathcal B(B \to D^{\ast}\,l\,\nu)$, where $l = (e,\,\mu)$, is more pronounced and the combined excess currently stands at $3.9\sigma$ level. If it persists and confirmed by future experiments, it would be a definite hint of new physics. In this context, we consider $B_c \to \eta_c\,l\,\nu$ and $B_c \to J/\Psi\,l\,\nu$ decays mediated via $b \to c\,l\,\nu$ charged current interactions and employ the most general effective Lagrangian in the presence of new physics to give prediction on various observables such as ratio of branching ratio, tau polarization fraction, and forward backward asymmetry for these decay modes.